my 17 yr old daughter has had a 2003 E53 3.0 since she was 15 and a half and got her permit. I was in the same boat as you. but had three rules.
1. didn't want to get her a car that I wouldn't drive or be seen in
2. had to be as safe or safer than anything I was driving
3. I had a budget, and was realistic about teenage driving and how long it might live.

you mention overhauling it and making it safe for the next 50k. most teenagers don't drive as much as we do with our daily commute. my daughter is only racking up maybe 5k a year compared to 10,12, or 15k like adults. so, your 50k estimate could be 10 years of driving? no way you could afford to maintain it that long. other issue is that probably 75% of kids now days total their first car before their 18. it happens. so you could only need your X5 to run 10k before it's not needed anymore. sounds like you've done your maintenance, and another 10k is pretty easy. and last, before you push the x5 to the curb, go shopping. find out what $7500 buys. not much. or you spend $10k, and he wrecks it the first year, and insurance pays you $6500. ouch. you're better off in my opinion to stick with what you know, put him in the x5 and hope for the best.